The present inventions relates to a prosthetic device for joints for implantation in humans and animals, the device comprising a joint body arranged between two fixing elements which are adapted to be connected to adjoining bone parts.
Several diseases cause destruction of joints, resulting in chronic pain and impaired movability. The problem is most pronounced in patients who suffer from chronic rheumatoid arthritis, but is also pronounced in, for instance, osteoarthritis (wear of cartilage) and in articular cartilage injuries after fractures and bacterial infections. In many of these cases the diseased and injured joint is replaced by an artificial joint structure.
However, experience has shown that too great an ambition to completely imitate the function of a normal joint in many cases results in failure. The artificial joint will easily be too complicated. For a rheumatic who has no joint function at all in, for instance, a hand, it is not necessary to aim at regaining a fully normal joint function via a prosthetic operation. The aim should instead be to obtain a painless joint with stability and a certain amount of movability, which makes the hand usable. Thus, the aim of the join structure must always be related to the patient""s needs.
The main difficulties when constructing artificial joints in e.g. hands have been (1) to provide an artificial joint with satisfactory properties and (2) to fix the artificial joint to adjoining bones in a satisfactory manner.
The artificial joint may be of a fairly simple design, for instance a flexible, integral silicone body which resists a certain degree of deformation. However, such a structure does not claim to imitate the structure of a normal joint, but instead a structure is desired, which is as simple as possible. The advantage of such a structure is above all the stability in the system and the inherent resilience. The great drawback, however, is limited mechanical strength and a tendency of the silicone towards unfavourable local tissue reactions.
More complicated artificial joints are as a rule based on some sort of artificial articular head in an artificial joint cavity, for instance in the form of a two-component device. The advantage of such a structure is that the movement pattern largely imitates the normal movement pattern of a joint and that a number of suitable biomaterials are available, inter alia by experience from knee and hip prostheses (for instance polyethylene, titanium, vitallium). Serious drawbacks of such structures are, however, the risk of wear products forming owing to friction, and the complexity of the structure, which in many cases results in undesired tissue ingrowth in the system. Moreover, some sort of mechanical motion rod is required to hold the two joint components together. Besides, there is no resilience at all in such a structure, which is a serious drawback in e.g. rheumatoid arthritis and similar states where the stretching tendons in many cases are considerably weakened.
There are two known main principles of fixing artificial joints to adjoining bones. According to one principle, no fixed anchoring is desired at all, but the shaft of the artificial joint is allowed to move back and forth in the marrow cavities of the adjoining bones, for instance when moving a hand and fingers. The advantage of this principle is that the movements in the artificial joint structure are spread over a long distance, and that the strain to which the material is subjected is thus reduced. A drawback is above all extensive foreign body reactions owing to movements and wear in the foreign material. This leads to the bone becoming thinner and increasing stiffness. Besides, fractures are common in the frequently used silicone, and the reconstructed joint easily becomes stiff.
According to the other principle for anchoring the artificial joint structure to adjoining bones, a fixed anchoring is desired. This was earlier effected by means of cement. However this principle suffers from serious drawbacks. For instance, in the case of hands, the bones therein have insufficient tolerance for heat development and toxic products from the cement, which has given disastrous results. A solution to this problem may be the cement-free so-called osseointegration principle which was introduced by professor BrAnemark. According to this principle, the anchoring elements are titanium screws which are inserted into the bone parts of the joint and there grow together with the bone, whereby the screws constitute permanent fixing points. The advantage of this technique is the absence of unfavourable tissue reactions and a probably permanent anchoring of the screws in the bone. Thus there are today satisfactory solutions to the previous problem of fixing artificial joints.
In plastic surgery of joints (arthroplasty) in e.g. hand surgery, use is today made of a limited number of methods based on different principles in respect of joint structure as well as fixing of the joint structure to adjoining bones. The most common method, Swanson""s silicone arthroplasty, is based on the first of the abovementioned principles of fixing. A solid body of silicone with two longitudinal shafts projecting in opposite directions from a flexible intermediate part replaces the diseased joint. The shafts are inserted in the marrow cavities of the long bone on both sides of the reconstructed joint. As mentioned above, the system, however, suffers from considerable drawbacks. The complications which in recent years have been described in connection with the use of silicone material as breast implants have resulted in this technique being seriously questioned, and its popularity seems to decrease more and more. The method definitely cannot be used on young rheumatics with an expected long length of life.
Another method used is based on a two-component device having an articular head in a joint cavity, and this device is fixedly anchored to adjoining bones. Attempts have been made to provide such anchoring by means of cement (for instance the St George technique). However, these attempts have not been successful in hand surgery since cement, owing to its heat development and toxicity, has resulted in tissue death and stiffness. The absence of resilience in the system besides results in bending contractures. Dislocations (luxations) are common since the articular head and the joint cavity easily slide apart. This technique is no longer in clinical use for reconstruction of finger joints. For wrist reconstruction, the method is of a certain value, but owing to increasing stiffness and tendency towards coming loose in the patient, the technique is not at all used in Sweden.
According to a further, more biologically oriented method, fixed anchoring points for the joint structure in the bone are established by means of the above-mentioned osseointegration method by inserting titanium screws (ad modum Branemark) into the bone tissue. Clinical experience shows that such screws remain immovable and fixedly anchored in the bone tissue. The advantage of this method is, inter alia, the absence of movable foreign material in the marrow cavities of the bones. In clinical application, use has up to now been made of a simple joint structure consisting of a solid body of flexible silicone. The movability of the joint structure thus is used in its entirety in that part of the structure which is located between the anchoring elements, i.e. the silicone component. The drawback of this method is that the stress to which this component is subjected will thus be very great, involving a risk of fatigue and damage to the silicone material. Fractures in the silicone material have been reported, and therefore the structure has not been considered sufficiently strong to allow introduction for general clinical use.
A further problem is to be found in the treatment of patients suffering from so-called thumb base osteoarthrosis in the wrist. This state involves osteoarthrosis, i.e. cartilage destruction, in the joint between the first metacarpal bone and the trapezoid bone (trapezium), optionally also between the trapezoid bone and navicular bone (scaphoideum). In this treatment, the trapezoid bone is usually removed and replaced by an interposed loop from an adjoining tendon. This treatment always results in a certain shortening in the system, and the clinical result varies too much to be quite satisfactory. In another method of treatment, the trapezoid bone is replaced by a silicone prosthesis, which is attached to the first metacarpal bone by means of a shaft inserted in the marrow cavity of the bone. Also this method of treatment, however, suffers from considerable drawbacks, among other things a tendency towards luxation of the prosthesis since there is no fixing to the navicular bone. Thus, for treating thumb base osteoarthrosis, no satisfactory replacement material is available.
WO 94/11606 (Chene et al) discloses an artificial joint system with two rigid components which can move relative to each other in a simple hinge mechanism. Inside the two components there are longitudinal channels which contain flexible elements, one in the form of a rod-shaped device extending through the two rigid components, and one being a helical spring at the far end of the one rigid component. This system allows movements in one plane only, which is moderated by the elasticity of the spring. The joint component in this structure is a hinge, whose mechanical properties are affected by the helical spring positioned at a distance from the hinge. This rigid system allows neither lateral deviations nor a shock-absorbing effect.
EP-A-O 454 645 (Medevelop AB) discloses an artificial joint mechanism which is entirely based on a solid body of thermoplastic adapted to be attached to fixed anchoring elements on each side of the joint structure. The joint body can be reinforced in various ways by means of longitudinal fibres, plates or netting structures. In one variant, use is made of an outer longitudinal helical spring, which serves only as reinforcement of the actual thermoplastic body and has no direct connection with the opposite bone part. In another embodiment, a helical spring with a diameter tapering in the direction of the attachment in one adjoining bone part is fitted in the thermoplastic joint body. However, the spring has no direct connection with the opposite bone part and is only intended to serve as an inner reinforcing complement to the solid joint body.
Swedish Patent Specification 8903838-4 (Volvo AB) discloses a finger joint prosthesis where at least one helical spring extending transversely of the direction of the joint constitutes the actual joint structure. The structure is of the xe2x80x9cclothes-pegxe2x80x9d type and is adapted to be attached to fixed anchoring elements on each side of the structure. The system yields satisfactory resilience, but there is no shock-absorbing effect and no possibility of lateral deviation and rotation in the system.
U.S. Pat. Specification 3,990,116 discloses a leaf spring structure, the longitudinal elements of which are, via shafts, fixed to the adjoining bones. The system can be enclosed in various types of artificial capsules. The joint structure permits movements in a single plane with resilience. However, the system has no shock-absorbing effect, and no capability of lateral deviation or rotation. Moreover, the friction between the spring leaves causes a risk of harmful wear products appearing.
Summing up, the existing prosthetic devices for joints suffer from a number of drawbacks, and there is a great need of replacement materials that satisfy all the desired requirements for stability, resilience, shock-absorbance, resistance to wear, sufficient capability of lateral deviation and rotation and a simple design.
The object of the present invention is to obviate the above-mentioned drawbacks that are associated with artificial prosthetic devices for joints that are available today.
This object is achieved by a prosthetic device for joints of the type mentioned by way of introduction, which has the features stated in the characterising clause of claim 1. Further features are defined in the subclaims.